The invention relates to a fuel injection pump with at least one pump piston that is controlled by edges on the piston and cylinder wall and which is driven with a constant stroke and is both axially and rotationally movable within a pump cylinder. The piston has a edge control recess in its outer surface, which is in constant communication with the operating chamber of the pump and which during the stroke of the piston connects the operating chamber with a chamber with a lower pressure-preferably the suction chamber of the pump-through a control bore in the wall of the pump cylinder to end the fuel delivery. The pump piston has an annular leakage oil collecting groove, that is separated from this edge control recess and which can be connected with the low pressure chamber through a leakage oil bore in the wall of the pump cylinder opposite the control bore, by means of leakage oil channel means formed in the outer surface of the piston diametrically opposite the recess, and extends in the direction of the operating chamber end of the pump piston.
A similar fuel injection pump is already known (FR-PS No. 1068783), whose pump piston has a leakage oil collecting groove machined into the outer surface of the piston and arranged separate from the edge control recess which determines the end of fuel delivery. Connected to this leakage oil collecting groove is a leakage oil channel that extends toward the operating chamber end of the pump piston and is formed as a flat area. The width and length of this leakage oil channel are such that at least during the effective useful stroke of the pump piston, there is a connection from the leakage oil collecting groove to the suction chamber of the pump by means of this channel and a leakage oil bore in the pump cylinder. This leakage oil channel being relatively wide, reduces the supportive portion of that part of the outer surface of the pump piston opposite the edge control recess to such a degree, that under very high injection pressures, the lubricating film, formed by the fuel between the surface of the pump piston and the wall of the cylinder bore of the remaining surface of the pump piston, is expelled. The increased surface pressure and simultaneous worsening of the lubrication causes an increased wearing of the pump piston. In addition, the very high injection pressures tend to bend the pump piston, which has been weakened on one side by the recess, whereby the above described wear of the pump piston is increased even more.
In a second embodiment of the above referred to known fuel injection pump, the leakage oil collecting groove and the leakage oil channel are formed by a single annular groove arranged across the piston to cooperate with the leakage oil bore in the wall of the pump cylinder during the useful stroke of the pump piston. As such, the single groove must be wide enough that the leakage oil return is effective in the desired useful stroke range of the pump piston. This wide cross groove leads to the same disadvantages as the above described flat area on the pump piston that serves as the leakage oil channel.